Method for improving fatigue strength in turbine blades



United States Patent 3,505,130 METHOD FOR IMPROVING FATIGUE STRENGTH IN TURBINE BLADES William Douglas Paul, Caledon, Ontario, Canada, assignor, by mesne assignments, to Orenda Limited, Toronto, Ontario, Canada, a company No Drawing. Filed May 29, 1967, Ser. No. 642,135 Claims priority, application Great Britain, June 13, 1966, 26,308/ 66 Int. Cl. C21d 7/00; C21f 1/00,- B24c 1/10 US. Cl. 148-11.5 9 Claims ABSTRACT OF THE DISCLOSURE A method for treating turbine blades fabricated from high tempreature alloys involving two cold working operations on the surfaces of the blades and a heat treatment between the cold working operations whereby a substantial increase in the strength of the blades is achieved.

BACKGROUND OF THE INVENTION The production of highly-stressed machine components usually involves considerable machining of forgings, wrought billits and the like. Generally speaking, the forging process gives better grain structure and improved physical properties of the material than do other processes. However, the increased machining required to produce the finished article results in greater production costs. Casting methods on the other hand have improved to such an extent that much less machine finishing is needed. The casting process, though, is still not conducive to the formation of the fine-grain sructure giving properties similar to those of forged products.

Grain size in castings can be reduced somewhat by controlling the cooling rate of the casting in the mould, but this is not a precise or easy method and varies in effectiveness with the size and intricacy of the casting. Another method used to maintain fine grain size is that of introducing inoculants into the melt. Inoculants work reasonably well with investment castings and particularly with small castings where casting temperatures can be kept relatively low. With more massive castings the inoculation method becomes less effective, presumably because the inoculants dissolve in the molten metal before solidification begins.

Cold working by processes such as shot-peening and cold rolling can, by introducing residual compressive stresses in the surface of a cast or Wrought material, im-

prove the fatigue strength and, in some cases, decrease susceptibility to stress corrosion. Cold working by itself, however, has been found not to yield these results uniformly with all materials, particularly in some of the cast super-alloys such as Inconel 713. Components which depend on residual stresses for improvement of fatigue strength may be used only where operating temperatures are not high enough to cause relief of the residual stresses.

It is well known that certain materials, such as some of the nickel-base group, which have had their surface layers cold worked and have then been heat-treated at a temperature sufficient to cause re-crystallization into a fine-grain structure at the surface, have improved fatigue strength. In US. Patent No. 2,920,007 to B. 0. Buckland a turbine blade for high temperature use is described which is cold worked to harden the surface of the metal to a depth of several mils. The patent teaches that the surface layer of the blade is then re-crystallized by heat treatment to a small grain size to improve the rupture and fatigue resistant properties of the outer protective layer. However, the alloys referred to in this patent are 3,505,130 Patented Apr. 7, 1970 wrought alloys, having been used as turbine blades only in the form of forgings. The procedure described in this patent is not nearly as successful with investment cast turbine blades. In any case, the treatment of wrought turbine blades in accordance with the teachings of this patent does not increase the fatigue resistance of the blades to the extent which would be most desirable.

SUMMARY It has now been discovered that the step of cold working the surface of the turbine blade a second time, after the heat treatment which causes re-crystallization in the surface layer, further considerably increases the fatigue strength of the turbine material.

In the method of the invention turbine blades of alloys designed for elevated temperature service are first cold worked at their surface layers such as by machining or shot-peening. The blades being treated are substantially in the shape desired for use before shot-peening or in the desired shape after machining if the first cold working step is accomplished by machining. After the first cold working step the blades are heat treated at a temperature or temperatures suflicient to cause re-crystallization of the coldworked surface layers into a fine-grained structure at the surface of the blades. The particular heat treatment depends of course on the particular alloy involved. After the heat treatment the blades are cold worked a second time at the surface thereof to further substantially increase the fatigue strength of the blades to re-introduce favourable residual stresses in the blades. The second cold working operation, not heretofore used in the art, which increases the turbine blade fatigue resistance to a considerable degree, is found, for example, to increase the fatigue resistance of superalloys, such as Inconel 713 alloy and Udimet 500 alloy, by in the order of an increase which is more than would be normally expected.

It is an object of the present invention to provide a procedure for substantially increasing the fatigue strength of turibne blades fabricated from high temperature alloys.

Other objects and advantages of the invention will be apparent from the following description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A turbine blade processed according to the invention is advantageously subjected first to preliminary heat treat ment operations to improve the mechanical properties thereof with any machining necessary, such as on the root structure, being carried out. The first cold working operation on the surfaces of the blade is then carried out with such cold working being necessary only on all sides of the root stem of the blade below the root platform thereof where improved fatigue resistance is particularly required. As afore-described shot-peening, carried out in accordance with techniques well known to the art, is an advantageous way of cold working the surfaces of the blades although machining of the surfaces also accomplishes this end. After the blade has been heat treated to cause re-crystallization of the cold-Worked surface layers into a fine-grained structure, the final cold working operation on the surface layers of the blade is advantageously carried out by way of a shot-peening operation in a manner similar to and in the same area as the first shotpeening operation.

The process of the invention is particularly attractive for components such as investment cast turbine blades and the like which are made from high-temperature superalloys such as Inconel 713 alloy and Udiment 500 alloy since, in general, their hot strength is such that it is not practical to break down the coarse-grained, cast structure by processes such as forging and rolling. Although in operation, gradual loss of the residual stresses introduced by the second cold working occurs in some parts of the blade, the service temperatures at the blade root, the critical area, are sufficiently low to ensure retention of the residual stresses over long periods. Notwithstanding the eventual loss of the residual stresses in the aerofoil area of the blade the fine-grain surface structure retains improved mechanical and thermal fatigue properties.

However, any blades fabricated from high temperature alloys which are work hardenable can be treated to advantage in accordance with the invention. Thus, even though a fine grain size is obtained at the surface of forged turbine blades of wrought alloys in accordance with prior art procedures, the three stage procedure of the invention involving first and second surface cold working operations with a re-crystallization heat treatment operation therebetween acts to improve the fatigue resistance of wrought turbine blades, fabricated from alloys such as, for example, Hastelloy B, Crucible 422, Nimonic 80 and Inconel X alloys, since the larger grain size in the substrate or core of the wrough blade inhibits creep.

For the purpose of giving those skilled in the art a better understanding of the invention the following illustrative examples are given:

EXAMPLE I A number of first stage turbine blades were investment vacuum cast from Udimet 500 alloy which is a nickelbase alloy containing 18% Cr, 18% Co, 4% Mo, 3% Al, 3% Ti and 2% Fe. The blades were given a standard heat treatment, first at 2100 F. for 4 hours in vacuum, followed by air cooling, and then heating for 2 hours at 1975 F. in vacuum, followed by air cooling. Four of the blades were then surface cold worked over the full root stem portion by machining and another four blades were cold worked on their surface, again over the root stem sections thereof, by short-peening in accordance with well known procedures. All eight blades were then heat treated by first heating at 1975 F. in vacuum for 2 hours and then at 1400 F. in vacuum for 16 hours. This heat treatment resulted in re-crystallization of the surface cold worked layer into a fine-grained structure. Two of the blades subjected to cold working by machining and two subjected to cold working by the shot-peening were then subjected to a second cold working treatment on the surface layer thereof by shot-peening on the root stern sections in a manner similar to the shot-peening first carried out. The second shot-peening treatment was omitted for the other four blades.

All eight blades were then fatigue tested by holding the roots of the blade stationary in a fixture while driving the airfoils electromagnetically at increasing amplitudes. The tests were run for 2X10 cycles at increasing amplitudes of 0.040" 0.044", 0.049, 0.054" and 0.060" until cracks developed, at which time a test was stopped. Thus, any one test was driven for 2X10 cycles at each amplitude until cracks developed. The results of these tests are set forth in Table A following:

TABLE A Final Reversals Blade First Cold Second Gold Amplitude at Final No. Working Working Reached (inch) Amplitude 1 Machined 0. 044 1, 734, 600 2 d 0. 049 1, 366, 800 0. 054 98,800 0. 054 687, 900 0. 060 l, 454, 100 O. 060 1, 033, 200 0. 060 462, 500 0. 060 1, 599, 840

It is readily apparent from the results shown in Table A that the blades subjected to the final surface cold working treatment had considerably improved fatigue strength over those blades not subjected to the second cold Working treatment. The fatigue strength of the four blades treated in accordance with the invention are seen to have had uniform high fatigue strength, all of them withstanding 2X10 reversals at the first four stress levels and well into the fifth stress level, whereas the blades partially processed, and not in accordance with the method of the invention, exhibited considerable scatter in properties with failures occurring in the second, third and fourth stress levels.

The operation of the method of the invention was also tested out on the nickel base Inconel 713 alloy. Speci- .nens were tested for endurance strength 1) in the as cast condition, (2) as cast and shot-peened, (3) as cast and treated in accordance with the process of the invention (shot-peened, heat treated and shot-peened), and (4) as cast and then heat treated and shot-peened (the first step of the process being deleted). Heat treatment where carried out was the same as that used in Example I. The results of these tests are set forth in Table B following.

R. Moore tests. Average of three R.

R. Moore tests. Average of two Cantilever beam tests. Cantilever beam tests.

(3) As cast and full three step 1 2 760 process. 000

(4) As cast, heat treatment 41,460

merit; and sliot-peened.

l The R. R. Moose is a well known fatigue test machine which employs a rotating beam test specimen.

2 These R, R. Moore fatigue test results were considered to be in error on the low slde since the high temperature heat treatment on these specimens caused a small amount; of distortion producing unusually severe vibration during the rotating beam fatigue tests.

The specimens subjected to the full three step process of the invention had considerably improved fatigue strength over the as cast and the as cast and shot-peened specimens. There is also seen to be an impressive improvement over specimens treated to heat treatment and shotpeening from the as cast condition, with one shot-peening operation being deleted.

It can be seen that the method of the invention results in substantially improved fatigue properties in items fabricated from superalloys and used in elevated temperature service and in particular in turbine blades made from uch alloys. It is found that the invention is useful for treating items made from any high temperature alloys which are work hardenable and it is found that the method of the invention can be used to advantage with wrought superalloys by permitting use of a higher-than-normal solution heat-treatment temperature. Though resulting in largerthan-normal grain size throughout the forging, it is possible to restore fine-grain structure at the surface, and thus the good, and in fact better, fatigue properties by treating the forged products in accordance with the invention.

What I claim as my invention is:

1. A method for increasing the fatigue resistance of turbine blades of work hardenable alloys designed for elevated temperature service, said blades having been formed to substantially the desired shape, which comprises first cold working at least part of the turbine blades only at their surfaces, heat treating the turbine blades at a temperature sufficient to cause recrystallization into a fine-grained structure at the cold worked surfaces of the blades and, after said heat treatment, cold working said blades a second time at the said surfaces thereof to increase the fatigue strength of the blades to a considerable degree.

2. A method as claimed in claim 1 wherein investment cast turbine blades are treated to increase their fatigue strength.

3. A method as claimed in claim 1 wherein wrought turbine blades are treated to increase their fatigue strength.

4. A method as claimed in claim 1 wherein the first cold working operation is carried out by machining the turbine blade.

5. A method as claimed in claim 1 wherein the first cold working operation is carried out by shot-peening the turbine blade.

6. A method as claimed in claim 4 wherein the second cold working operation is carried out by shot-peening the turbine blade.

7. A method as claimed in claim 5 wherein the second cold working operation is carried out by shot-peening the turbine blade.

8. A method as claimed in claim 7 wherein shot-peening of the turbine blade is directed only on all sides of the root stern of the blade below the root platform thereof.

' 9. A method as claimed in claim 2 wherein the first References Cited UNITED STATES PATENTS 2,560,973 7/1951 Martin et al. 148-12 2,782,135 2/1957 Richardson 1481 1.5 2,920,007 1/ 1960 Buckland 14811.5 X

OTHER REFERENCES Metals Handbook, vol. 2, 1964, pp. 398-403, and 488.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 14812, 39 

